1. Field of the Invention
The present invention relates to an electrostatic chuck which is used in an apparatus for manufacturing semiconductor devices or liquid crystal devices and mounts thereon a wafer such as a semiconductor wafer and a liquid crystal glass substrate.
2. Prior Art
Conventionally, in manufacturing processes for semiconductor devices, in order to carry out processes such as film-forming processes and etching processes on wafers such as a silicon wafer with high precision, the wafer is required to be held while properly remaining in high degree of flatness. For such a holding means, electrostatic chucks have been proposed.
The electrostatic chucks can easily fix a wafer thereon with a high degree of water flatness which is required for various processes such as film forming or etching processes. The chucks are also applicable even in a vacuum so that they have been widely incorporated in a wide variety of film-forming devices and etching devices.
Here, a further improvement in integration degree of semiconductor elements in semiconductor devices, there have been strong demands for stable performance characteristics, an improvement in production yield, and an increase in the number of wafers which can be processed per unit time. For this reason, it is required that in etching or forming a film, the wafer is heated to a target temperature as quickly as possible to improve the uniform heat distribution on an overall wafer.
Therefore, the mounting surface of the electrostatic chuck for supporting a wafer thereon is provided with grooves or recesses thereon and a gas inlet communicating with the grooves or recesses for supplying an inert gas such as He or Ar to the grooves or recesses. As a wafer is chucked onto the mounting surface, a narrow space formed between the lower surface of the wafer and the groove or recesses of the chuck is filled with an inert gas through the gas inlet so that thermal transfer characteristics between the wafer and the mounting surface on the electrostatic chuck are improved, obtaining a uniform heat distribution on a heated wafer.
Moreover, with respect to configuration of grooves or recesses formed in the mounting surface, Japanese Patent No. 2626618 discloses an chuck structure, as shown in FIG. 2, including: a gas inlet 34; a plurality of radial grooves 35 communicating with the gas inlet 34; and multiple annular grooves 36 formed concentrically with virtually equal intervals, centered on the gas inlet 34, which are allowed to communicate with the respective radial grooves 35, with the upper face of the electrostatic chuck 31 other than the radial grooves 35, annular grooves 36 and gas inlet 34 serving as the mounting surface 33 of the wafer W.
Furthermore, as shown in FIGS. 3A and 3B, JP-A-10-56054 discloses a disc-shaped electrostatic chuck 41 having: a gas inlet 44; a plurality of radial grooves 45 communicating with the gas inlet 44; and an annular groove 46 communicates with the ends of the respective radial grooves 45, in which the other upper portions of the electrostatic chuck 41 are prepared as a mounting surface 43 for supporting a wafer W than the radial grooves 45, the annular groove 46 and the gas inlet 44 and the mounting surface 43 is subjected to a blasting process to be formed into a surface with protrusions and recessions, as shown in FIG. 3B.
As shown in FIG. 4, JP-A-7-153825 discloses a chuck structure having: a plurality of fine protrusions 52 dotted on the upper surface of an electrostatic chuck 51 in a disc shape with an annular protrusion 57 formed on the peripheral edge of the upper surface, in which the top faces of the fine protrusions 52 and the annular protrusion 57 serve as a mounting surface 53 for supporting a wafer W with a plurality of gas inlets 54 being formed on the upper surface.
Moreover, JP-A-9-213777 discloses an electrostatic chuck constituted by a combination of annular gas grooves with radial gas grooves, and, as shown in FIG. 5, another electrostatic chuck 61 constituted by a combination of annular gas grooves 66 with circular gas recesses 67.
JP-A-7-18438 discloses an electrostatic chuck is formed with a plurality of annular gas grooves on its surface, and protrusions formed inside a circular gas recess.
Furthermore, as shown in FIG. 6, U.S. Pat. No. 5,761,023 discloses an electrostatic chuck 71 having double annular gas grooves 76; a circular gas recess 77; fine dotted protrusions 78 distributed inside the area of the circular gas recess 77; and an annular rib protrusion 79 located between the double annular gas grooves 76 to be separated into two grooves.
However, in the case of the electrostatic chuck 31 having a mounting face configuration as shown in FIG. 2, with respect to the area of the mounting surface 33 surrounded by the adjacent two radial groups 35 and annular groove 36, the area in the peripheral portion is larger than that in the center portion and therefore the amount of heat transfer from the mounting surface 33 to the wafer W in the peripheral portion is greater than that in the center portion, resulting in wafer temperature at the peripheral portion of the wafer W higher than that at the center portion, with degradation in a uniform heat distribution over the surface of a wafer W.
Moreover, the electrostatic chuck 41 having amounting surface 43 shown in FIGS. 3A and 3B, the mounting surface 43 is a surface including protrusions and recesses among them, which has a small contact area with the wafer; therefore, the low heat quantity to be transmitted from the top face of the fine protrusions to the wafer extends a time required to heat the wafer thereon to a predetermined temperature and the subsequent degreases the number of processed wafers per unit time.
Furthermore, in the electrostatic chuck 51 having the mounting surface 53 shown in FIG. 4, the contact area to the wafer W is as small as 1 to 10% so that the quantity of heat, transmitted from the top faces of the fine protrusions 52 and the annular protrusions 57 to the wafer, is small; therefore, the time required for heating the wafer to a predetermined temperature is taken longer, which does not increase the number of processed wafers per unit time, i.e. wafer process speed.
As shown in FIG. 5, the electrostatic chuck 61, which is constituted by the annular gas groove 66 and the circular gas recess 67, is sucked by the annular protrusion 69 located between the annular gas groove 66 and the circular gas recess 67, with the result that the gas pressure in the gas groove section becomes greater to cause a deformation in the wafer due to the gas pressure, resulting in a failure in sucking the wafer in an even manner and the subsequent temperature difference on the wafer surface.
Further, an electrostatic chuck 71 has been proposed in which gas pressures in two zones, prepared by an annular gas groove 76 and a circular gas recess 77 shown in FIG. 6, are supplied with the gas pressure from the annular gas groove 76 being set greater so that the heat conductivity is made greater to prevent a temperature decrease on the periphery of the wafer W and also to make the wafer temperature distribution uniform. However, since the annular gas groove 76 on the peripheral portion is divided by the annular protrusion 79; therefore, although the temperature difference is made smaller in a direction orthogonal to the annular gas groove 76, the temperature difference is merely made smaller within the span of the width of the annular gas groove 76, with the result that microscopic low temperature portions tend to occur in response to the annular protrusion 79 along the length direction of the gas groove on the periphery of the wafer. Upon manufacturing a recently-developed high density semiconductor element, for example, a semiconductor element having a width of a circuit line of 65 nm, the above-mentioned low temperature portions tend to generate a defective semiconductor element, resulting in a reduction in the yield of the semiconductor elements in the wafer.